Two classes of drilling tools are used in oil drilling, namely, roller-cone bits and fixed-cutter bits. Roller-cone bits have moving parts whilst fixed-cutter bits are mono-block without any moving parts.
Drill bits with fixed cutting structures fall into four sub-categories according to the nature of their cutters. Such cutters can be made of polycrystalline diamond compacts (PDC), natural diamonds, thermally stable polycrystalline diamond (TSP) or a metal-based material impregnated with diamonds or other abrasive particles. This last category is generally termed as “impregnated” bits.
A method of manufacturing a drill bit with a fixed cutting structure is described in U.S. Pat. No. 7,621,349. The fixed cutting structure is made from a metal-based material impregnated with diamonds or other abrasive materials.
It is necessary to be able to assess the level of wear of drill bits with fixed cutting structures so that it is possible to determine their effectiveness when used in a drilling operation. However, it can be difficult to obtain a wear assessment for drill bits, in general, in terms of a wear level at stages of operation. Cutting structure wear is normally determined in accordance with standards set by the International Association of Drilling Contractors (IADC). The relevant standard, the IADC dull grading system, determines the amount of wear of a drill bit in accordance with the height of its original cutting structure. For PDC cutting structures, the height is determined by the diameter of the PDC cutters themselves compared to their original diameter. For impregnated bits, cutting structure height is determined as the blade height measured on the nose area parallel to the main bit axis, the nose area being defined by the tip of the blade profile. A grading system of “1” to “8”, representing 0% to 100% wear is used in accordance with the measured PDC cutter or blade height depending on the type of cutting structure. However, as the determination of wear depends on a measurement, it can be inconsistent and therefore unreliable, particularly as when worn, it is impossible to determine the original blade height for impregnated bits.
U.S. Pat. No. 6,167,833 describes a wear indicator for use with rotary drilling tools. The wear indicator is incorporated into a leading surface of the tool and comprises at least one area of visually distinct material that is arranged to indicate progressive wear of the leading surface. The visually distinct material is different to the material from which the drilling tool is mainly composed. The wear indicator may be a embedded in outer surface portions in a graduated way. For example, the wear indicator may comprise: a step arrangement, in which each step indicates the progression of wear of the bit; a wedge arrangement, in which portions of the wedge are exposed in accordance with wear of the tool in which it is embedded; or a plurality of fins, in which the number of exposed fins is an indication of the amount of wear. The wear indicator may comprise stainless steel, brass, aluminium, tungsten, graphite or a ceramic material.
However, the wear indicator described in U.S. Pat. No. 6,167,833 suffers from the disadvantage that, unless one is familiar with the particular wear indicator and how it progresses during the lifetime of the tool in which it is embedded, the determination of the amount of wear may be less than accurate.